(1) Field of the Invention
The present invention relates to a paper feed structure for use in an image forming apparatus such as a copier, printer facsimile machine, etc., which performs image forming by selectively feeding and conveying paper sheet by sheet from a stack of paper stored at the bottom of the apparatus, transferring an unfixed developer image onto the paper that is fed at a higher position, fusing and fixing the unfixed developer image to the paper at a further higher position, and discharging the paper after fixing upwards or sidewards.
(2) Description of the Prior Art
Conventional image forming apparatuses such as copiers, printers, etc., perform an image forming process comprising the steps of: developing the electrostatic latent image of an original image, written or formed on the photoreceptor drum, with a developer; transferring the developer image to a sheet of paper (recording paper) fed from a paper feed cassette; fixing the developer image thus transferred to the recording paper, by fusing and pressing while nipping it between a heat roller and pressing roller; and discharging the paper.
In this image forming process, multiple kinds of recording paper are used. Of these various kinds of recording paper, postcard paper that is small-sized and thick (usually having a basis weight of 128 g/m2) is not used in a large amount. For this reason, there is not much demand for products with a dedicated paper feed tray for postcard paper, so usually postcard paper is fed through a manual paper feeder mechanism that is separately configured from the paper feed portion with paper feed trays.
Japanese Patent Application Laid-open Hei 4-327446 (Patent document 1) discloses an image forming apparatus that uses a manual paper feeder mechanism. Typically, as described in this patent document 1 a paper conveyance path for a manual paper feeder mechanism is laid out so as to join into the paper conveyance path from the cassettes, at a position before an idle roller for controlling the timing at which paper is delivered to the photoreceptor drum.
Recently, image forming apparatuses having a compact design configuration, front access (meaning a configuration in which almost all control operations of image forming can be made from the front side of the machine) and a buildup system have been developed.
Illustratively, the image forming apparatus includes: a paper feeder that stores paper at the bottom of the apparatus and selectively delivers paper, sheet by sheet, from the paper stored therein; a printing portion for transferring the toner image, which was formed by developing an electrostatic latent image (written by laser beam) on a photoreceptor drum, to the fed paper; a fixing portion for fusing and fixing the unfixed developer image onto the paper; and a paper discharge portion for discharging the paper with the developer image fused and fixed thereon, to a space above the fixing portion or to the side-wall portion of the apparatus.
Nowadays, many kinds of image forming apparatuses have been developed which have a paper feed portion, a printing portion, a fixing portion, a paper discharge portion and an original reading portion arranged one over another from bottom to top of the apparatus.
The image forming apparatus of this type employs a conveying system that has an essentially vertical paper feed path along which recording paper is conveyed upward from the paper feed portion. It is common practice that the aforementioned manual paper feeder mechanism is arranged at a position between the paper feed portion and the printing portion on the exterior of the apparatus.
The path of the paper that is conveyed from the manual paper feed mechanism of this type needs to join to the aforementioned paper conveyance path directed upward, at a position before the printing portion (before the idle roller), so the manually fed paper has to pass through a conveyance path that has a markedly large curvature, compared to the aforementioned conveyance path.
When small-sized sheets such as postcard paper, etc., being thick, hence high in rigidity, are fed to the conveyance path from this manual feeder mechanism, the paper of this kind may stagnate in the conveyance path and may cause the so-called jam problem (jam: paper becomes stuck and unable to move).
The same problem will occur when, other than postcard paper, special thick paper (having a basis weight of 128 g/m2 to 250 g/m2) that is thicker than usual paper (having a basis weight of about 50 to 100 g/m2, for example) is used.